The invention relates to a method for communication by radio, in which a subscriber station is handed over from a first to a second network-based radio station.
In radio communication systems, messages, for example containing voice information, image information, video information, SMS (Short Message Service), MMS (Multimedia Messaging Service) or other data, are transmitted between a sending and a receiving station via a radio interface using electromagnetic waves. In this case, depending on the specific configuration of the radio communication system, the stations may be various kinds of subscriber stations or network-based radio stations, such as repeaters, radio access points or base stations. In a mobile radio communication system, at least some of the subscriber stations are mobile radio stations. The electromagnetic waves are emitted at carrier frequencies which are in the frequency band provided for the respective system.
Current mobile radio communication systems are often in the form of cellular systems, e.g. based on the GSM (Global System for Mobile Communication) or UMTS (Universal Mobile Telecommunications System) standard, with a network infrastructure comprising base stations, devices for inspection and control of the base stations and other network-based devices, for example. Aside from these cellular, hierarchic radio networks organized over a wide area (superlocal), there exist wireless local area networks (WLANs) with a radio coverage area with generally much greater physical limitation. Examples of various standards for WLANs are HiperLAN, DECT, IEEE 802.11, Bluetooth and WATM.
To achieve the most efficient transmission of data possible, the entire available frequency band can be broken down into a plurality of subbands (multicarrier method). The idea on which the multicarrier systems are based is to transfer the initial problem of transmitting a broadband signal to the transmission of a plurality of narrowband signals. Splitting the available bandwidth into a plurality of narrowband subbands allows a much higher level of granularity for the data transmission when the data to be transmitted are distributed over the various subbands, i.e. the radio resources can be distributed over the data to be transmitted or over the subscriber stations with a great deal of fineness. An example of a multicarrier transmission method is OFDM (Orthogonal Frequency Division Multiplexing), in which pulse forms which are approximately rectangular over time are used for the subbands. The frequency spacing of the subbands is chosen such that, in the frequency domain, at that frequency at which the signal in one subband is evaluated, the signals in the other subbands have a zero crossing. The subbands are therefore orthogonal with respect to one another.
If a radio communication system contains a plurality of network-based radio stations, the mobility of subscriber stations means that it is advantageous if it is possible for a subscriber station to be handed over from a first to a second network-based radio station without the need for the subscriber station's communication to be aborted. In the case of what is known as a hard handover, the subscriber station communicates with the first network-based radio station prior to the handover and communicates with the second network-based radio station after the handover; accordingly, the subscriber station is not connected to both network-based radio stations simultaneously.